If I recall, grand juries only hear the evidence presented by the prosecutor who is appearing in front of them for the explicit purpose of getting an indictment.
The defendant does not have an opportunity to provide evidence or tell his side of the story at that time.
One of the resident legal minds here at FR could no doubt give some better insight on the issue.
You pretty much nailed it.
It’s certainly possible for a grand jury to be used for political purposes to achieve a political outcome and the members not realizing or believe it.
It all depends on how selective the DA is in presenting evidence. I’m guessing here that the DA was highly selective in both evidence presented AND interpretation of the law presented.